Image forming devices such as electrostatic photocopiers, laser beam printers, and plain paper facsimiles use, as a method for developing an electrostatic latent image on an image retaining body such as a photosensitive drum, a two-component developing method using a two-component developing agent comprising a magnetic carrier and nonmagnetic toner, and a one-component developing method using a one component developer comprising only toner.
The two-component developing method entails such problems as a carrier moving on a photosensitive drum, causing transfer defects, and the carrier further moving on transfer paper, causing scratches on a fuser roller of a fuser device. On the other hand, the one-component developing method often causes charging of toner by friction, leading to the problem of image defects such as fog frequently occurring due to toner charging defects.
Thus, as a development method to resolve these problems, a so-called hybrid development method has been proposed. The hybrid development method is a development method such that a two-component developing agent comprising nonmagnetic toner and a magnetic carrier is maintained on a magnetic roller periphery surface, and the two-component developing agent is brought in contact with a development roller periphery surface to form a toner layer consisting only of the nonmagnetic toner on the development roller, and the toner layer is sprayed on an electrostatic latent image on a photosensitive drum, thereby executing the development. Because this development process is executed when the development roller and photosensitive drum are in a non-contact state, it is advantageous in terms of inhibiting degradation of image quality.
In this hybrid development method, regardless of the coverage of the formed image, the supply of toner from the magnetic roller to the development roller is always executed across the entire periphery surface of the development roller, and further, toner not consumed by the development process and remaining on the development roller is usually removed by an electrical method such as applying a reverse bias voltage. Alternatively, a conventional development device may be configured so that toner not consumed by the development process and remaining on the development roller is removed by a physical method employing toner release means disposed downstream in the development roller rotation direction.
Further, in a different prior art, there is a hybrid development method using as a development device that removes nonmagnetic toner remaining on a development roller without application of a large mechanical force thereupon, thereby inhibiting occurrence of ghosting, wherein a form factor used for a magnetic carrier is 130 or more. Form factor is a value indicating particle shape and surface conditions, and is calculated by the following formula: [(carrier perimeter)2/(projected area of the carrier )×(¼)×100]. The degree of projections and recesses on a particle surface is represented by this form factor, and a carrier having a form factor of, for example, 130 or more has a large degree of unevenness, and indicates a nonspherical carrier.
In a hybrid development method, as described above, regardless of the coverage of a formed image, supply of toner from the magnetic roller to the development roller is always executed across the entire periphery surface of the development roller, and toner not consumed by the development process and remaining on the development roller is removed by an electrical method such as application of a reverse bias voltage. Repeatedly execution of this removal operation causes magnetic carrier to be contaminated with fine powder toner (so-called toner spent) and by free external additives, causing the problem of degraded magnetic carrier charging properties. Such degradation of magnetic carrier occurs particularly when an image with low coverage is repeatedly developed, causing toner charging defects and the problem of low-charge toner scattering within a development apparatus (toner scattering).
Further, as in the former prior art described above, when toner release means is pressed against the development roller and toner remaining after the development processing is removed, because toner release means is always pressed thereagainst, the development roller may wear, and the invention is not appropriate for uses requiring device durability.
Further, because in the hybrid development method, each time image formation is executed, a toner layer is formed on the development roller by a magnetic brush on the magnetic roller comprising a magnetic carrier and nonmagnetic toner, and after development, the toner layer remaining on the development roller is removed by the magnetic brush, the opportunities for magnetic carrier and nonmagnetic toner to be in contact increase considerably when compared to the case of a conventional two-component developing method, and replacement of the nonmagnetic toner retained by the magnetic carrier has to be performed more often. Therefore, with hybrid development, problems such as a large amount of toner spent, described above, and the peeling off the coating layer from the magnetic carrier surface easily occur.
Further, as in the second prior art described above, because when a nonspherical carrier having greater surface projections and recesses is used as a magnetic carrier in a two-component developing agent, the contact ratio between magnetic carriers increases, and sufficient charge can be applied, reaching the magnetic carrier at the magnetic brush tip, allowing a greater electric field to work in order to move the nonmagnetic toner between the development roller and magnetic roller, enabling prevention of ghosting. However, because the magnetic carrier has a nonspherical shape and has an uneven surface, fluidity of the two-component developing agent degrades, entailing new problems such as increase of toner spent through repetition of the image forming process, and accelerated peeling of the magnetic carrier coating layer.
Thus, it is an object of the present invention to provide a two-component developing agent used for image formation in a hybrid development method and capable of forming an image while preventing degradation of a magnetic carrier and consistently forming a high-quality image.